Efficient activation of sulfite by Zn/Mn co-doped γ-Fe2O3 for Iohexol degradation: The combined effects of oxygen vacancies and metal redox cycles

Sulfite-activated advanced oxidation processes (AOPs) represent a cost-effective approach for organic pollutant degradation. In this study, the Zn/Mn co-doped γ-Fe₂O₃ catalyst (Zn/Mn-γ-Fe₂O₃) is developed through synergistic modulation of oxygen vacancies (Vo) by Mn(II) and Zn(II), in order to efficiently activate sulfite for the degradation of iohexol (IOX). The Zn doping ratio of 0.05 is found to be optimal for achieving enhanced IOX degradation efficiency. In the combination system of Zn/Mn-γ-Fe₂O₃ and sulfite system, the removal of IOX increases with higher Vo concentrations, where SO₄^•− serves as the dominant reactive species and HO^• plays a supporting role. The Vo-rich surface endows Zn/Mn-γ-Fe₂O₃ with lower electrochemical impedance and enhanced electron transfer efficiency. According to the theoretical calculations, Vo can strengthen electron transfer and increase the adsorption capability between catalyst and sulfite via the synergistic Fe-Mn interaction accelerating Fe(II)/Fe(III) redox cycling through Mn doping. Valence state analysis confirms that dual redox cycles of Fe and Mn collectively promotes sulfite auto-oxidation and radical generation. Notably, the good performance in diverse real water matrices demonstrates the significant practical potential of the Zn/Mn-γ-Fe₂O₃/sulfite system within sulfite-based AOP systems.